Reactive N additions and their effects in microbial activity and soil respiration under a Nothofagus glauca (Phil.) Krasser forest of Chile

Increases in available soil N under forests can affect several ecosystem functions including soil CO₂ fluxes and microbial activity. Most of the studies, concerning reactive N deposition in natural ecosystems, are located in the northern hemisphere, meanwhile little is known in southern forest ecosystems, including deciduous Nothofagus forests. We evaluated the effect of reactive N additions (N-NO₃ broadcasted under 20 trees at a rate of 400 Kg ha^-1 yr^-1) in soil CO₂ fluxes (Rs, μmol CO₂ m^-2 s^-1), microbial biomass C (MBC), and related ecophysiological indexes, in the most septentrional N. glauca forest of Chile (19S 310464 S, 6213139 W). Twenty additional N. glauca trees, with no N additions, were used as controls. After seven months of N-application, soil respiration was significantly higher (p<0.001) under the N-fertilized trees than in the control trees. Soil MBC, the ratio between MBC and soil organic C (q_mic), and the ratio between cummulative basal respiration and SOC (q_min) decreased significantly (p<0.05) in the N-enriched trees, but only after two years of N application. Basal respiration and the metabolic quotient qCO₂ were not affected by N enrichment. Even though soil respiration increased, N enrichment inhibited microbial activity. Temporal variations in MBC, basal respiration, and q_mic were also found and were associated to water availability.